Popular Science Monthly/Volume 27/October 1885/The Energy of Life Evolution, and How it Has Acted
|THE ENERGY OF LIFE EVOLUTION, AND HOW IT HAS ACTED.|
By Professor EDWARD D. COPE.
HAVING pointed out in a previous essay the lines of descent of vertebrata which have been brought to light by paleontological investigation, I propose to produce in the present article some evidence as to the nature of the forces which have been actively at work in effecting those changes of structure which constitute the evolution of one type of animal from another. We can obtain this evidence by comparing the successive steps of each line with one another. We thus learn the nature of the modifications, and can, as the case may be, surmise or demonstrate their causes. It is evident that, without the genealogical lines which paleontology presents, it is impossible that our hypotheses on this subject can rest on any solid basis. With these lines completed, we will be able, on the other hand, ultimately to reach a demonstration in most if not all the cases which present.
In the first place, there has long been before the world a theory to account for these changes, and that is the doctrine of use and disuse, propounded by Lamarck. He believed that the use of a part of an animal caused it to grow larger, and that in consequence of disuse a part would grow smaller or become extinct. Another theory is frequently spoken of, as though it accounted for the origin of changes, and that is the Natural Selection of Darwin. While naturalists are generally agreed as to the importance of this principle in modifying the results of the creative energy, but few of them regard it as explaining in any way the origin of the changes with which it deals. In the very nature of things, "selection" can not act until alternatives have been presented. And Mr. Darwin and others, who treat of natural selection as though it were a cause of new structures, always premise by admitting that "there is in organic beings a natural tendency to variation." It is in accounting for this variation that the Lamarckian hypothesis is useful, and probably expresses a great law of organic nature. In the same way most of those who write of the "influence of the environment" (which Lamarck, by-the-way, fully considered), as though it embraced the causes of evolution, forget that the energy which impresses an animal from this source could have no effect unless the animal possess some impressibility or capacity for response. And they also often forget that an animal capable of free movement is able to modify its environment very materially.
There is one element of weakness in the Lamarckian theory of use. This is that use implies the presence of something to use, or the existence of a usable part or organ. It is thus incompetent to explain the origin of such a part ab initio, although it may account for the details of its structure, as its segmentation, branching, etc. So I have added to use the energy of effort, and the Lamarckian theory, completed, can be characterized as the theory of the origin of species, by effort, use, and disuse. A prominent cause of change of structure may be here referred to; and that is, change in consequence of excess or diminution of growth-energy, due to the action of use and effort in disturbing its equilibrium. That is to say, that excessive growth in one place has caused diminished growth in another, and vice versa. This derivative hypothesis explains the origin of many structures which are not useful to the possessor, and of others which appear to be positively injurious. Such characters can not, of course, be accounted for by the direct action of effort and use, which are necessarily directed toward beneficial ends. There are also numerous characters, chiefly of an ornamental nature, as color, etc., which are the direct result of the physical impress of the environment, as temperature, light, humidity, movements of the medium, etc., which are only influenced by the animal as it places itself, by its movements, within or without the range of their influence. In spite of these facts, I believe the movements and habits of animals to lie at the foundation of their principal characters, and that the superstructure, be it due to whatever cause it may, rests upon that foundation. It will now be well to take a look at the evidence in favor of or against these theories, as presented by the science of vertebrate paleontology. A few examples will suffice.
In the first place, I will select an illustration of the effects of use on the articulations of the limbs and feet of the mammalia. I take first the ankle-and wrist-joints. In the ruminating animals (ox, deer, camel, etc.) and in the horse, among other living species, the ankle-joint is a very strong one, and yet admits of an extensive bending of the foot on the leg. It is a treble tongue-and-groove-joint; that is, two keels of the first bone of the foot, the astragalus, fit into two grooves of the lower bone of the leg, the tibia, while between these grooves a keel of the tibia descends to fill a corresponding groove of the astragalus. Such a joint as this can be broken by force, but it cannot be dislocated. Now, in all bones the external walls are composed of dense material, while the centers are spongy and comparatively soft. The first bone of the foot (astragalus) is narrower, from side to side, than the tibia which rests upon it. Hence the edges of the dense side-walls of the astragalus fall within the edges of the dense side-walls of the tibia, and they appear to have pressed into the more yielding material that forms the end of the bone, and pushed it upward, thus allowing the side-walls of the tibia to embrace the side-walls of the astragalus. Now, this is exactly what would happen if two pieces of similar dead material, similarly placed, should be subjected to a continual pounding in the direction of their length tor a long period of time. And we can not ascribe any other immediate origin to it in the living material; but the probability of such origin is more probable in such substance, because of the perpetual waste and repair which are going on, and because of the wonderful power which we so often see in growth, in repairing damages, and in providing for new conditions in cases of accidents. This inclusion of the astragalus in the tibia does not occur in the reptiles, but appears first in the mammalia, which descended from them.
The same active cause that produced the two grooves of the lower end of the leg produced the groove of the middle of the upper end of the astragalus. Here we have the yielding lower end of the tibia resting on the equally spongy material of the middle of the astragalus. There is here no question of the hard material cutting into soft, but simply the result of continuous concussion. The consequence of concussion would be to cause the yielding faces of the bones to bend downward in the direction of gravity. If they were flat at first they would begin to hollow downward, and a tongue above and groove below would be the result. And that is exactly what has happened. Without exception, every line of mammalia commenced with types with an astragalus which is flat in the transverse direction, or without median groove. From early tertiary times to the present day, we can trace the gradual development of this groove in all the lines which have acquired it. The upper surface becomes first a little concave; the concavity gradually becomes deeper, and finally forms a well marked groove.
The history of the wrist-joint is similar. The surface of the forearm bones which joins the fore-foot is in the early tertiary mammalia uniformly concave. In the ruminating mammals it is divided into three fossæ, which are separated by sharp keels. These fossæ correspond with the three bones which form the first row of the carpus or palm. The keels correspond to the sutures between them. The process has been evidently similar to that which has been described above as producing the side-grooves in the end of the tibia. The dense walls of the sides of the three bones impinging endwise on the broad yielding surface of the fore-arm (radius) have gradually, under the influence of countless blows, impressed themselves into the latter. On the contrary, the surface above the weaker lines between the bones not having been subject to the impact of the blows, and influenced by gravity, remains to fill the grooves, and to form the keels which we observe.
There is another striking instance of the same kind in the feet of mammalia; that is, in the development of the keels and grooves which appear at the articulation of the first set of bones of the toes (metapodials) with the bones of the second set (phalanges). These keels first appear on the posterior side of the end of the first set of bones, projecting from between two tendons. These tendons, in many mammals, contain two small bones, one on each side, which act like the knee-pan, and resemble it in miniature, which are called sesamoid bones. These tendons and bones exercise a constant pressure on each side of the middle line, when the animal is running or walking, and this pressure, together with the concussion with the ground, appears to have permitted the protrusion of the middle line in the form of a keel, while the lateral parts have been supported and even compressed. The reptilian ancestors of the mammals do not possess these keels.
|Fig. 2.||Fig. 3.||Fig. 4.|
|Fig. 2.—Hind-foot of Primitive Cameloid Poëbrotherium labiatum, showing grooved astragalus and first toe-bones without keel in front at lower end. (From Colorado.)|
|Fig. 3.—Hind-foot of Three-toed Horse (Protohippus sejunctus) (from Colorado), showing grooved astragalus, and trace of keel on front of lower end of first bone of middle toe.|
|Fig. 4.—United First Bones of Two Middle Toes of Deer-Antelope (Cosoryx furcatus), showing extrusion of keel on front of lower end. (From Miocene of Nebraska.)|
Fig. 5.—Part of the Vertebral Column of the Fossil Batrachian Eryops megacephalus, one fourth natural size, showing segmentation, the left side.
as those of the horse, the pig, and of the ruminants, the ends of the toes are applied to the ground, and are covered with larger hoofs, which surround the toe, and the cushion is nearly or quite dispensed with. These animals are especially distinguished by the fact that their metapodial keels extend entirely round the end of the bone, dividing the front, as well as the end and back, into two parts. This structure would seem to be a result of the greater force of the impact resulting from use of the legs, experienced by the end and front of the bone, which receives the blows.
I cite one more instance of the effect of use on the structure of the skeleton, and this time from the vertebral column. In a certain order of Batrachia from the Permian formation (the Rhachitomi), the bodies of the vertebræ are curiously segmented. Instead of a solid slightly modified from a cylinder, as in most vertebrates, we have three pieces. One of these is of nearly the form of a segment cut from an apple or orange having a crescentic outline and wedge-shaped section. The sharp edge is concave and is directed upward. On one side of each of the horns of this crescent a rhombic piece is applied, which, widening upward, supports the separate arch of the vertebra. These three pieces when together leave a central vacancy, which becomes, when all the vertebræ are placed end to end, a canal or tube. This was occupied by what is known as the chorda dorsalis, which is the central axis of the body of the simplest vertebrates, and is present in the early embryonic stages of all. In the growth of a reptile or mammal this flexible chorda is replaced by the bony vertebral bodies. The osseous material appears in the membranous sheath which covers it, and, gradually encroaching on it, first cuts it into segments and then fills it entirely. In the rhachitomous batrachia this process is not completed, as the chorda remains more or less entire; and the ossification of its sheath and substance is laid down in the three segments already described.
The two segments visible from one side of the column form two wedges with their apices together, and their bases one up and the other down. Now, if a person who wears a coat of rather thick material will examine the folds of his sleeve as they are produced on the inner side of his arm, he will see a figure nearly like that of the segments of the vertebral column described. The folds will correspond to the sutures, and the interspaces to the bony segments. He will find that the spaces are lens-shaped, or, when viewed in profile, wedge-shaped, with the apices together. This arrangement results from the necessary mechanics of flexure to one side. In flexure of a cylinder like the sleeve, or like a vertebral column, the shortest curve is along the line of the greatest convexity of the cylinder. Here is the closest folding of the sheath, and here, consequently, the lines of fold in soft material, or fracture in hard material, will converge and come together. That is just what they do in both the sleeve and the rhachitomous vertebral column, the only difference being that in the animal
Fig. 6.—The Folds on the Inner Side of a Coat-sleeve, which correspond with the lines separating the segments of the vertebræ of the Eryops. The letter i is the basal segment or intercentrum; the p corresponds with the lateral segment, the pleurocentrum; and n represents the basal part at the upper or neural arch, which rests on p and i.
it is exhibited on both sides, and on the sleeve on only one side. This difference is, of course, due to the fact that the animal can bend himself in both directions, while the arm only bends in one direction.
It results from the above observations that the structure of the rhachitomous vertebral column has been produced by the movements of the body from side to side, as in swimming, during the process of the deposit of mineral material in and around the chorda dorsalis. Here we have another convincing proof that use and effort have produced animal structure.
Instances like the above can be cited from many departments of zoölogy wherever paleontology has pointed out the lines of descent. I will not cite them further, but will draw some conclusions which are necessary and which are of general. interest.
It is evident that use and effort imply some kind of movement on the part of the animal which puts them forth. Hence I have called the theory which holds that the structures of animals are the results of their movements the theory of kinetogenesis (from kineo, I move, and genesis). This theory is supported not only by the class of facts which I have adduced, but by another large class of a different kind, which demonstrate the alternative proposition, viz., that disuse is followed by loss and extinction of parts. This may be inferred from the very degenerate character of most animal parasites taken in connection with their embryology. The absence of limbs, of segments, of sense-organs, and even of more important vital organs observed in various parasites, is very remarkable. It is equally remarkable that the history of the development of such animals shows that in their earlier stages they are not parasitic, and possess many organs which are wanting in the adult. This brief history, condensed into the life of each individual parasite, no doubt, as in all growth-history, merely repeats that of the species as a whole. It teaches us that the ancestors were active, independent swimmers or fliers, as the case may have been, and that by the adoption of parasitic habits they ceased to use the many parts, which consequently dwindled and disappeared. This conclusion is sustained by paleontology wherever evidence can be obtained from that source. This is most evident in the history of the reptiles. Many forms of lizards of the present period are known to display curious defects. Such are the absence of some or all of the toes; of some or all of the limbs; of the eyelids; of the dermal folds about the eyes; and, finally, of the eyes themselves. Paleontology shows that these are not ancient or primitive types which survived, but that they are modern. Now, nearly all such lizards have habits which involve the least possible use of the limbs and of the sense of sight; they are subterranean, and many of them inhabit ants' nests and devour those insects for food. It is evident that here is a kind of parasitism, and its consequences are of the most marked character.
The nature of animal movements may now claim our attention. These, we know, are referred to two general divisions, the automatic and the voluntary. The popular definition of these classes of movements is that, in the latter case, choice or preference, and therefore will, is supposed to be exercised, and that in the former there is no such quality displayed. The automatic movements are called mechanical, and may be performed unconsciously, as the movements of the heart and digestive system, etc. Rigorously, however, the so-called voluntary acts do not proceed from any freedom or will proper on the part of an animal. They appear to do so, because they display intelligence of a higher order than the automatic acts, although it is true that intelligent design is not wanting from the latter either. Moreover, the so-called voluntary acts may be unconsciously performed, and the automatic may be consciously performed, as in the winking of the eyes and breathing, when attention is directed to them. So, then, the classification into conscious and unconscious is quite independent of that into voluntary and automatic. As the term voluntary is misleading, the word ratiocinative has been substituted for it.
The relations of animal acts may, then, be considered as follows: Automatic acts display design for the well-being of the animal, but are invariable in their action, not changing immediately in adaptation to new or modified needs. Ratiocinative acts, on the other hand, are performed in accordance with circumstances as they arise, and are not rhythmical or invariable in their action. Their existence implies the presence of a certain development of mind, which the automatic acts do not so obviously display. Ratiocinative acts are very common in animals, as those who observe them can always testify. The automatic acts increase in relative importance as we descend the scale of being, but as they also display a general beneficial design it is not possible to draw the line between them and the ratiocinative. In fact, the one passes into the other by the well-known process which I call cryptonoÿ, as will presently be explained. If acts affect structure, it is evident, that if the acts are beneficial, the structure they produce must be so also. From what has preceded, it is also evident that the more intelligent acts will produce correspondingly more beneficial structures than the less intelligent. But since these changes are only effected by long-continued movements on the part of an animal, it is clear that an act is likely to become automatic before it can become an important cause of evolution of animal forms. The history of animal movements has probably been as follows:
Fig. 7.—Skull of Coryphodon elephantopus, two ninths natural size; a corner removed from the skull so as to display the small brain-cavity at the base, (from New Mexico.)
Protoplasm presents certain movements, of which contractility is one, which will respond to certain stimuli under proper conditions of nutrition and temperature. It may perform movements which cause a simple mass of it to change its location in a fluid medium. Such acts, however, lack the element of design, or adaptation to the needs of the animal, whether they be constant or temporary ones. In order to display this property it is necessary that sensation should exist, and that this should be pleasurable or painful, in order to produce a determinate movement to increase the one or escape the other. This is the basis of all design, for without consciousness there can be no design. Just where this consciousness first displays or displayed itself in living things it is not possible to know at present with certainty, but its first exhibition was probably in the pain of hunger. The first designed act was, then, the taking of food. The first structure was, therefore, also some kind of arrangement for seizing and surrounding food. This having been performed, another set of functions had its birth, one which was destined to follow all new experiences, and in turn to dominate all later acts. This is the memory of the act and of its consequences, which remains as the basis of mind. An impress once, made on consciousness is not lost, because it has modified the molecular structure of some part of the living material in a way as yet unknown to a us. The movement which results from this memory is the first designed act, and this also affects structure, and produces the first motor link between mind and body, as the first stimulus & perceived produced the first sensory link. From this time onward the law of use and effort has its way. Its first result is to build a mind-machine, or nervous system, or its equivalent. This kind of building has evidently preceded all others in time, and its latest and highest product is the human brain. The evidence of vertebrate paleontology places this statement beyond the stage of mere hypothesis. We have learned that, with a few minor exceptions, the brains of the vertebrata, and especially of the mammalia, have greatly increased in complexity and in size with the lapse of geological time. This is illustrated by the accompanying figures of the brain-chambers or casts of brain-chambers of two ancient and one late Tertiary mammals. The Coryphodon elephantopus is the largest animal of the three, and the Phenacodus primævus, of which the skeleton was figured in the preceding article, is a little smaller than the Procamelus occidentalis, the third species. The last is the latest species in time, and is one of the ancestors of the existing camels and llamas. The much greater size and complexity of the brain, and especially of the cerebral hemispheres, as compared with the two other species, are striking.
We have now the beginning of an explanation of the element of design in the movements of animals. But, first, the explanation is necessary to account for the long-continued automatic acts which have changed the hard and apparently fixed structures of so many of the higher forms of life.
Automatic and unconscious ratiocinative acts are the product of conscious ratiocinative acts by the process of cryptonoÿ already referred to. This process is one of the most wonderful which the field of science presents to our contemplation. It is simply this: that when a brain, or other organ of consciousness, has once acquired an habitual movement, consciousness disappears from that act, and it enters the unconscious and generally automatic stage. This demonstrates two things: First, that consciousness is not necessary to a designed act which has become a habit, no matter how complicated that habit may be; second, that consciousness does reside in matter which has not acquired habits, and which therefore does not yet possess the structure which makes such habits possible.
We now have a true theory of the influence of the environment on an animal. Sensation being understood, the animal proceeds to adapt itself to its surroundings by the adoption of appropriate habits, from which appropriate structures arise. Without such response on the part of the animal, the greater part of the world would have remained uninhabited by all but the lowest forms of life, and these too might have been extinguished. From the simplest temporary methods of defense and protection, animals have developed the habits of laying up stores, of building houses, of the arts of the chase, of migrations over wide territories. There can be no doubt that the constant exercise of the mind in self-support and protection has developed the most wonderful of all machines, the human brain, whose function is the most wonderful of phenomena, the human mind. And the acts of other parts of the organism, which have been the outcome of this process, have produced the varied structures which to-day constitute the animal kingdom.
It is thus shown to a demonstration, by means of the principle of kinetogenesis, that evolution is essentially a process of mind. The source of the consciousness, which is back of it, is at present an unsolved problem. That it has existed and does exist, there can be no question, and there is no sufficient reason for supposing that it will not continue to exist.
- This subject is more fully treated of in the "American Naturalist" for January 1884.
- The word human is emphasized because it is not yet proven that protoplasm is the only possible physical basis of mind.